This invention relates to electric power supply apparatus of a type in which by means of semiconductor devices a first unidirectional input voltage is switched so as to reverse its polarity at a relatively high freqeuncy. The switched voltage is fed to a transformer the output of which is rectified to produce a second unidirectional output voltage, usually differing from the input voltage. Such a system is usually referred to as a voltage converter, and will be so called in this specification. The unidirectional input to the converter may itself be obtained by rectification of an alternating current supply.
Voltage converters of the above type are known, see for example U.S. Pat. to Crowe et al No. 4,164,014 which discloses what are known as half-bridge and push-pull converters and U.K. Pat. No. 1,459,885 which discloses what is known as a full-bridge converter. Although these inverters have been known for some little time, it has proved difficult to provide a regulated output from them. This has usually meant that if a regulated output was required it was necessary to supply the converter with a regulated input, or to use a very crude form of control known as burst-fire control by switching the converter off and on at intervals so that over a long period of time the output could be considered stable. Such a control system is disclosed in the above-mentioned British Pat. No. 1,459,885.